In order to rapidly and accurately detect a status of current gear ratio steps of automobile manual transmission, a method has been disclosed which shift position sensors to detect respective current gear ratio steps are respectively provided to gear ratio change mechanism of the transmission. The method has a merit to accurately detect the current gear ratio step, but has a demerit to make a product cost to increase due to complex structure with the same number of the shift position sensors as number of the gear ratio steps.
Meanwhile, a gear ratio indicating apparatus as described in patent document 1: Publication No. JP2008-39111, has also been disclosed which, instead of providing the shift position sensors to the gear ratio change mechanism, a current gear ratio step is supposed to be determined based on signals outputted by two rotational speed sensors which are disposed in input shaft side and output shaft side, respectively.
A gear-position determination apparatus as described in patent document 2: Japan Patent No. 4404096, has also been disclosed which a current gear ratio step is supposed to be determined based on ratio of an engine revolution and a vehicle speed calculated from signals outputted by an engine revolution sensor and a vehicle speed sensor, respectively.
In the gear ratio indicating apparatus as described in the patent document 1, there has a merit to prevent increase of product cost with respect to whole transmission system, because a determination of current gear ratio step is conducted based on the two rotational speed sensors disposed on the input and output shafts and consequently, the structure of electrical signal system becomes to be simplified.
However, when the revolution is not stable due to twist vibration and backlash generating in drive train at time right after a synchronizer has completed to engage with a target gear or when a synchronizer has begun to move to engage with a target gear, it is impossible to calculate an accurate gear ratio based on the respective revolutions of the input and output shafts measured by those rotational speed sensors. In the result, a controller such as engine control unit possibly erroneously detects the current gear ratio step and, in worst case, possibly makes the indicating apparatus flash intermittently on and off. Furthermore, in a jumping upshift, i.e., 2 steps jumping upshift from 2nd to 5th gear ratio step, at the moment of passage of revolutions corresponding to intermediate gear ratio steps (3rd and 4th gear ratio steps in this case), the controller erroneously detects each of the intermediate gear ratio steps as a current gear ratio step as well.
Moreover, in the gear ratio determination apparatus as described in the Patent document 2, a current gear ratio step is supposed to be determined by differential value of vehicle speed in addition to ratio of the engine revolution and vehicle speed.
However, the engine revolution becomes to be temporarily unstable immediately after the clutch has disengaged from the engine. Consequently, the apparatus in the Patent document 2 has a problem that it is hard to accurately detect the current gear ratio step of the transmission. The apparatus additionally has another problem that it is hard to prevent the controller from erroneous detection of the current gear ratio step as long as the differential value of vehicle speed does not exceed a given threshold.